1. Field of the Invention
The present invention relates to a system in which a plurality of multipoint connected satellite stations are connected via a down line and an up line to a master station. More particularly, it relates to a technique of retraining a modulator and demodulator (MODEM) provided in each of the satellite stations.
2. Description of the Related Art
In a multipoint connected communication system, plurality of MODEMs provided in satellite stations (hereinafter referred to as a satellite station MODEM) are multipoint connected via a down line and an up line to a MODEM provided in a master station (hereinafter referred to as a master station MODEM). Access to terminal equipment provided at each satellite station is carried out according to a polling procedure from a host computer provided in the master station and, in turn, data from the terminal equipment to which access is carried out is modulated in the corresponding satellite station and sent to the master station.
In this communication operation, once the master station starts sending the polling data, a demodulation unit in the satellite station MODEM, which receives the polling data via the down line, is brought to a status of normally receiving data (data normally-receiving status). This is because the satellite station MODEM per se is not conscious of the kind of data being received, i.e., it cannot discriminate user data such as the polling data. In this regard, the master station sends a training signal to each of the satellite station MODEMs prior to the sending of the polling data. Upon receipt of the training signal, each satellite station MODEM initializes an automatic gain controller (AGC), an automatic equalizer (AEQ), an automatic carrier phase controller (CAPC), and the like, provided in the corresponding demodulation unit. After the initialization, the satellite station MODEM receives and demodulates user data such as the polling data.
However, where a line trouble or abnormality due to instantaneous breaking or the like occurs in the down line, seen from the demodulation unit in the satellite station MODEM, through which data communication is normally carried out, a disadvantage arises in that operation parameters of the AGC, AEQ, CAPC, and the like, in the satellite station MODEM are changed to abnormal values. As a result of the disadvantage, it takes long time to restore demodulation processing or operation by the satellite station MODEM to its normal status after the trouble concerned is recovered.
In this regard, a technique is strongly demanded in which, even if a trouble occurs in the down line, the demodulation processing by the satellite station MODEM can be quickly restored to its normal status after recovery of the trouble.